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Chapter 30: Plant Diversity II – the Evolution of Seed Plants

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Chapter 30: Plant Diversity II – the Evolution of Seed Plants Chapter 30: Plant Diversity II – The Evolution of Seed Plants 1. General Features of Seed-Bearing Plants 2. Survey of the Plant Kingdom II A. Gymnosperms B. Angiosperms The 10 Phyla of Existing Plants Chapter 29 Chapter 30 1. General Features of Seed-bearing Plants Key Adaptations for Life on Land Plant life on land is dominated by seed plants due to the following 5 derived characters: 1. SEEDS 2. REDUCED GAMETOPHYTES 3. HETEROSPORY 4. OVULES 5. POLLEN Advantages of Seeds A seed is a sporophyte embryo surrounded by nutrients packaged in a protective seed coat which provides the following advantages for the embryo: • the fruit surrounding the seed can facilitate its dispersal over long distances • the embryo can survive for years in a dormant state until conditions are favorable for germination Fireweed seed • nutrients to sustain the embryo during early growth Advantages of Reduced Gametophytes Seed plants have microscopic gametophytes that are fully contained within the sporangium of the sporophyte. This provides the following advantages: • the reproductive tissues of the sporangium protect the gametophyte from environmental stresses (e.g., UV exposure, loss of moisture, extreme temperature) • the sporophyte can provide nourishment to sustain the gametophyte PLANT GROUP Ferns and Mosses and other other seedless Seed plants (gymnosperms and angiosperms) nonvascular plants vascular plants Reduced, Independent Reduced (usually microscopic), dependent on Gametophyte Dominant (photosynthetic surrounding sporophyte tissue for nutrition and free-living) Reduced, dependent Sporophyte on gametophyte for Dominant Dominant nutrition Gymnosperm Angiosperm Sporophyte Microscopic female (2n) gametophytes (n) Microscopic female inside ovulate cone Sporophyte gametophytes (n) (2n) inside these parts of flowers Gametophyte Microscopic (n) male Example gametophytes (n) inside these parts Microscopic of flowers male gametophytes (n) inside pollen cone Sporophyte (2n) Sporophyte (2n) Gametophyte (n) Advantages of Heterospory Most seedless plants are homosporous – produce one type of spore that develops into a bisexual gametophyte. Seed plants are heterosporous and produce 2 types of spores: Megasporangium Female on megasporophyll Megaspore gametophyte Eggs Microsporangium Male on microsporophyll Microspore gametophyte Sperm This provides 2 key advantages: 1. male & female gametophytes can mature at different times avoiding self fertilization and increasing genetic diversity 2. a separate female gametophyte can better support a developing embryo Egg Production in Ovules Seed plants are unique in containing the megasporangium within the parent sporophyte surrounded by a protective integument. Immature The complete ovulate cone structure – Integument (2n) Female Seed coat megaspore within gametophyte (n) Megaspore (n) megasporangium Spore Spore wall Egg nucleus wall within the (n) integument – is Discharged Food called an ovule. Megasporangium sperm nucleus supply (2n) (n) (n) Pollen Male Pollen tube Embryo (2n) Micropyle grain (n) gametophyte (n) (a) Unfertilized ovule (b) Fertilized ovule (c) Gymnosperm seed Pollen and Sperm Production Microspores develop into multicellular pollen grains – a male gametophyte surrounded by a protective outer layer containing sporopollenin produced by the sporophyte. • pollen grains protect the male gametes – sperm – and facilitate their dispersal without the requirement for water • unlike seedless plants, the sperm contained in pollen grain are not flagellated and gain access to an egg at pollination through a pollen tube Five Derived Traits of Seed Plants Reduced Microscopic male and Male Summary of the Key gametophytes female gametophytes gametophyte (n) are nourished and protected by the Female Derived Traits of sporophyte (2n) gametophyte Heterospory Microspore (gives rise to a male gametophyte) Seed Plants Megaspore (gives rise to a female gametophyte) Ovules Integument (2n) Ovule (gymnosperm) Megaspore (n) Megasporangium (2n) Pollen Pollen grains make water unnecessary for fertilization Seeds Seeds: survive Seed coat better than unprotected spores, can be Food supply transported long distances Embryo 2A. Survey of the Plant Kingdom II Gymnosperms Nonvascular plants (bryophytes) Seedless vascular plants Gymnosperms Angiosperms Gymnosperm Characteristics Gymnosperms are at least 300 million years old according to the fossil record and were the dominant group of land plants in the Mesozoic era, with many still existing today. • gymnosperm means “naked seed” which refers to the exposed seeds produced on modified leaves (sporophylls) of cones • cones are a type of strobilus, a collection of sporophylls It takes approximately 3 years for an ovulate cone to produce mature seeds! Ovule The Life Cycle Megasporocyte (2n) Ovulate cone of a Pine Integument Pollen cone Microsporocytes Pollen Megasporangium Mature (2n) grains (n) sporophyte (2n) • most conifers such as (2n) MEIOSIS Germinating MEIOSIS pines produce both pollen grain ovulate (female) Microsporangia Seedling Microsporangium (2n) cones and pollen Surviving (male) cones megaspore (n) Archegonium Seeds Female gametophyte • conifer pollen grains Food Sperm have an aerodynamic reserves Seed nucleus (n) coat (2n) morphology and reach Pollen tube Key megasporangia Embryo Haploid (n) (new sporophyte) FERTILIZATION Diploid (2n) through the air (2n) Egg nucleus (n) Gymnosperm Phyla 4 of the 10 plant phyla are gymnosperms: • CYCADOPHYTA • GINGKOPHYTA • GNETOPHYTA • CONIFEROPHYTA Phylum Cycadophyta Cycads have large cones and palm-like leaves. • produce flagellated sperm unlike most seed plants • widespread during the Mesozoic period, most modern species are endangered Cycas revoluta Phylum Ginkgophyta There is only one living species in this phylum: Gingko biloba • produces flagellated sperm like the cycads • a popular ornamental tree in southern California Ginkgo biloba Phylum Gnetophyta This phylum contains only 3 genera: Gnetum, Ephedra, and Welwitschia Welwitschia Gnetum Ovulate cones • some species are tropical and others thrive in deserts Welwitschia Ephedra Phylum Coniferophyta • this is the largest gymnosperm phylum with ~600 known species • most conifers are evergreens Douglas fir that carry out photosynthesis year round Bristlecone pine Common juniper Sequoia European larch 2B. Survey of the Plant Kingdom Angiosperms Nonvascular plants (bryophytes) Seedless vascular plants Gymnosperms Angiosperms Characteristics of Angiosperms All angiosperms (literally “enclosed seeds”) or flowering plants belong to the phylum Anthophyta and have 2 key adaptations: 1. Flowers as sexual reproductive structures 2. Seeds enclosed in fruits which aid in seed dispersal Flowers Stigma Carpel Stamen Anther Style Flowers are modified Filament Ovary shoots containing up to 4 types of modified leaves: • sepals that enclose and protect the flower • petals to attract Petal pollinators Sepal • stamens – the male reproductive organs Ovule • carpels – the female Receptacle reproductive organs Variations in Flower Structure Location of Stamens Flower Symmetry and Carpels • flowers of any given Sepal Common species of angiosperm holly Radial flowers may have radial or symmetry with bilateral symmetry (daffodil) stamens Fused petals Stamens • flowers of any given Nonfunctional species may also be Carpel stamen complete (have all 4 Bilateral flower organs) or symmetry (orchid) Common incomplete (lacking at holly flowers least one flower organ with carpels Fruits develop from Fruits the ovary wall and ▼ Mechanisms that aid in the dispersal disperse seeds by of seeds by a variety ▼ Tomato explosive action of methods: ▼ Ruby grapefruit • animals disperse ▼ Wings seeds in edible fruits ▼ Nectarine • wind disperses some seeds (e.g., dandelion) ▼ Seeds within • hitchhiker fruits act berries and as barbs to stick to other edible ▼ animals passersby Hazelnut fruits • some fruits burst open when dry to disperse seeds ▼ Barbs ▼ • (e.g., peas) Milkweed Microsporangium Carpel Anther Microsporocytes (2n) Mature flower on Angiosperm sporophyte plant Microspore (2n) MEIOSIS (n) Generative cell Ovule with Tube cell Life Cycle megasporangium (2n) Tube nucleus Male gametophyte (in pollen grain) Ovary (n) Germinating MEIOSIS Pollen grains seed Stigma Pollen tube Megasporangium Surviving (2n) Sperm Embryo (2n) megaspore Integuments (n) Tube Endosperm (3n) Seed Micropyle nucleus Seed coat (2n) Antipodal cells Polar nuclei Style Female in central cell gametophyte (embryo sac) Synergids Egg (n) Zygote (2n) Nucleus of developing Egg endosperm nucleus (n) (3n) FERTILIZATION Key Haploid (n) Discharged sperm nuclei (n) Diploid (2n) Carpel Possible Angiosperm floats Stamen History • the oldest angiosperm fossil dates to ~140 million years ago 5 cm (a) Archaefructus sinensis, • angiosperms a 125-million-year-old fossil dominate the fossil record as of ~100 (b) Artist’s reconstruction of million years ago Archaefructus sinensis and still dominate the world today Evolutionary Links with Animals Many animals and angiosperms have coevolved due to close relationships that may be adversarial or mutually beneficial: • angiosperms have evolved defenses in response to herbivores that would eat them • angiosperms and their animal pollinators have evolved characters to reinforce their mutualistic symbioses Angiosperm Phylogeny Living gymnosperms MicrosporangiaFossil and Bennettitales (contain microspores)molecular Amborella evidence Water lilies dictate the Most recent common Star anise and ancestor of all living relatives
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